1. Field of the Invention
This invention relates to an image pick-up tube used for a television camera, etc., and in particular to the structure of electrostatic deflecting electrodes in a magnetic focusing and electrostatic deflecting (hereinbelow abbreviated to MS) image pick-up tube.
2. Description of the Related Art
In an MS image pick-up tube proposed heretofore an electro-magnetic coil disposed so as to surround its vacuum envelope (glass tube) focuses an electron beam and two pairs of electrostatic deflecting electrodes formed on the inner surface of the glass tube deflect the electron beam.
FIG. 1 is a cross-sectional view illustrating the construction of a prior art MS image pick-up tube. An electron gun 7 consisting of a cathode 71, a first grid 72, a second grid 73 and an adsorption electrode 74 for return electron beam is disposed at one end within the glass tube. On the second grid 73 is formed a beam disk electrode having an extremely small aperture for forming a fine electron beam. The electron gun 7 generates the electron beam 8. At the other end of the glass tube 1 are arranged a photoconductive target 3 scanned with the electron beam 8 and a mesh electrode 4. This target 3 is disposed on a face plate 2. On the inner surface of the glass tube 1 are formed electrostatic deflecting electrodes 5 generating deflecting electric fields in order to scan the target 3 in the horizontal and vertical directions with the electron beam 8. A focusing coil 6 generating a focusing magnetic field for focusing the electron beam 8 on the surface of the target 3 is disposed on the outer periphery of the glass tube 1 so as to surround the glass tube 1. A cylindrical electrode 9 is disposed between the mesh electrode 4 and the deflecting electrodes 5. The mesh electrode 4 and the cylindrical electrode 9 are connected with each other so that they are at a same potential. The potential difference between the cylindrical electrode 9 and the deflecting electrodes 5 constitutes an electrostatic lens. This electrostatic lens is called a collimating lens and acts so as to remove radial landing errors of the electron beam deflected by the deflecting electrodes 5. Further the mesh electrode 4 forms a decelerating electric field between the target 3 and the mesh electrode 4 and enables the scanning with a low-speed electron beam.
The deflecting electrodes 5 are formed by depositing a conductive film by vacuum evaporation on the inner surface of the glass tube and cutting it e.g. by means of a laser beam into 4 zig-zag patterns separated from each other. These deflecting electrodes 5 are called pattern yokes. FIG. 2A is a development scheme of the pattern yokes seen from the inside of the glass tube 1. Such a zig-zag shaped pattern yoke is disclosed in U.S. Pat. No. 2,830,228 to Schlesinger. FIG. 2B is a scheme illustrating these pattern yokes seen from the target 3 of the glass tube 1, where the thickness of the electrodes is neglected. The line B.sub.1 B.sub.2 connecting the upper apices M of a zig-zag shape of the pattern yokes in FIG. 2A is in the form of a spiral extending from one end to the other end of the pattern yokes on the inner surface of the glass tube, while rotating around the center axis O of the glass tube. The rotation angle of this line B.sub.1 B.sub.2, i.e. the center angle &lt;B.sub.1 OB.sub.2 formed by the lines OB.sub.1 and OB.sub.2 in FIG. 2B connecting the points B.sub.1 and B.sub.2, respectively, where the two ends of the pattern yokes intercept the line B.sub.1 B.sub.2, with the axis O of the tube is called twist angle and designated by .omega.. In the example illustrated in the figure the twist angle .omega. is equal to 180.degree.. The ordinate of FIG. 2A represents the twist angle measured from the point A.sub.1, A.sub.2. It is disclosed in U.S. Pat. No. 3,666,985 to Schlesinger that the pattern yokes have a certain twist angle .omega.. The pitch between two adjacent upper apices of the zig-zag shape of the pattern yokes is designated by L and the number of repetitions by n. Then the total length of the pattern yokes is nL.
Among the pattern yokes the electrodes H.sup.+ and H.sup.- are horizontal deflecting electrodes, to which horizontal deflecting voltages +V.sub.H /2 and -V.sub.H /2, respectively, superposed on a bias voltage E.sub.C3 are applied, forming a deflecting electric field in the horizontal direction. The electrodes V.sup.+ and V.sup.- are vertical deflecting electrodes, to which vertical deflecting voltages +V.sub.V /2 and -V.sub.V /2, respectively, superposed on the bias voltage E.sub.C3, forming a deflecting electric field in the vertical direction.
It is disclosed in U.S. Pat. No. 4,663,560 granted May 5, 1987, that in such an image pick-up tube the most suitable twist angle of the pattern yokes is 30.degree. for the purpose of increasing remarkably the uniformity of the resolution. The twist angle is disclosed as being uniform along the axis of the tube.
Further, it has been known that such an MS image pick-up tube can be used under a condition where the voltage applied to the mesh electrode is higher with a larger twist angle. When the voltage applied to the mesh electrode is high, beam bending can be small. Beam bending refers to a phenomenon where the trajectory of the electron beam is bent towards clear parts on the target 3 on which an optical image is projected, thereby producing local distortions of the image and lowering of the resolution. Consequently it is desirable to use twisted pattern yokes in order to ameliorate the uniformity of the resolution or to reduce the beam bending. However, even if these means are used, in an MS image pick-up tube having high resolution characteristics owing to a high voltage applied to the mesh electrode, the potential difference between the mesh electrode and the deflection electrodes cannot be increased significantly because of the strength of the collimating lens constituted by the potential difference therebetween. There are also limits in lowering the DC voltages applied to the deflecting electrodes and in reducing focusing electric power or deflecting electric power.